Norms of reaction and adaptational value considered in a tree breeding context

Abstract

Separability of genetic from environmental effects on target traits is a central concern of each breeding program. This requires studies of norms of reaction, i.e., responses of genotypes to environmental conditions that potentially participate in the modification of the trait. Yet, in addition to this modifying environmental condition, achievement of the breeding goal can be affected by a second component of the environment, which decides upon the adaptedness of the desired trait expressions. These adaptive environmental conditions may also vary and may be associated in various ways with the modifying conditions with the result that the desired phenotype is adaptively inferior to less desired phenotypes. Therefore, it is important to know the prerequisites under which a consistent phenotypic superiority guaranteed by separability of the genetic effects transforms into consistent adaptational superiority relations among genotypes. After having recalled the system analytic basis of adaptational processes, this problem is tackled with the help of a paradigm model in which photoperiod dynamics modifies growth conclusion in forest trees and first frost decides upon the adaptedness of the time of growth conclusion in terms of the realized annual growth increment. In this paradigm, maximization of growth increment constitutes both the breeding goal and adaptational valuation of the trait "growth conclusion" under the adaptational condition "first frost." The central result of the analysis rests on the definition of a fictitious genotype, whose time of growth conclusion equals the time of first frost, and which thus characterizes the association between modifying and adaptive conditions that guarantees maximum adaptedness. The result then states that separability of the genetic effects on growth conclusion within the set of genotypes enlarged by the fictitious genotype implies consistent adaptive ranking among genotypes. The implications for common breeding practice and its evolutionary consequences are discussed.